Conventionally, a magnetic resonance imaging apparatus (hereinafter, referred to as “MRI (Magnetic Resonance Imaging) apparatus”) is an apparatus that takes an image of the inside of a subject by using a magnetic resonance phenomenon, and includes various units for performing a scan, such as a static magnetic-field magnet that generates a static magnetic field in a scan region, a gradient magnetic-field coil that applies a gradient magnetic field to the subject, and a radio-frequency coil that receives a magnetic resonance signal from the subject who is applied with the gradient magnetic field.
Some of such units generate heat during a scan. Particularly, the gradient magnetic-field coil remarkably generates heat during a scan because a pulse current is repeatedly supplied in accordance with a pulse sequence. For this reason, an MRI apparatus usually includes a chiller for cooling a gradient magnetic-field coil. The chiller includes, for example, a heat exchanger or a circulation pump, and cools the gradient magnetic-field coil by circulating a coolant, such as water, through a cooling pipe that is provided to the gradient magnetic-field coil (for example, see Patent Document 1).
The gradient magnetic-field coil is sometimes provided with an iron shim for correcting ununiformity in a static magnetic field inside a scan region in some cases. When the temperature of the gradient magnetic-field coil varies, magnetic permeability of the iron shim changes due to an influence of a variation in the temperature. When the magnetic permeability of the iron shim changes, a change appears in static magnetic field uniformity in the scan region, and particularly change in the center frequency is remarkable.
It is known that a variation in the center frequency can be an obstacle to fat suppression or a cause of an artifact in an image. Therefore, to obtain an image of stable quality, it is required to suppress variations in the temperature of the gradient magnetic-field coil. Usually an MRI apparatus suppresses variations in the temperature of a gradient magnetic-field coil by detecting the temperature of the gradient magnetic-field coil by using a temperature sensor, and changing the flow rate of a coolant to be flowed into a cooling pipe in accordance with a detected temperature change.
Patent Document 1: JP-A 2006-311957 (KOKAI)
In addition to a gradient magnetic-field coil, some of units included in an MRI apparatus, such as a static magnetic-field magnet and a gradient magnetic-field amplifier (G-amplifier), need to be cooled, and such units are each to be cooled by a chiller similarly to the gradient magnetic-field coil. FIG. 12 is a schematic diagram for explaining cooling of units of an MRI apparatus according to the conventional technology.
For example, as shown in the figure, it is assumed that the MRI apparatus includes a gradient magnetic-field coil, a unit A, and a unit B, as units each of which needs to be cooled. In such case, the gradient magnetic-field coil, the unit A, and the unit B are each provided with a cooling pipe through which a coolant, such as water, is circulated. The cooling pipe of each of the units is connected to a chiller that is placed, for example, outdoors, and a coolant at a constant temperature (for example, 20° C.) is circulated by the chiller. Each of the units is then cooled by circulating the coolant inside the unit through the cooling pipe.
As described above, according to the conventional MRI apparatus, a chiller for cooling units that need to be cooled is often shared among the units in many cases. Consequently, when the flow rate of the coolant is changed to stabilize the temperature of the gradient magnetic-field coil, the temperature of the other units of which temperature does not need to be changed also changes correspondingly, thereby influencing functions of the units.
A cooling pipe for circulating a coolant is usually made from metal, such as copper, as a material; however, when the flow rate of the coolant flowing through the pipe is changed, erosion of an inner wall of the cooling pipe progresses more quickly, as a result, the life of the cooling pipe becomes shorter.
For such reasons, in order to obtain a stable quality as a primary purpose, it is required to suppress variations in the temperature of the iron shim without changing the flow rate of a coolant to be flowed into the cooling pipe.